This invention relates to correction of pattern on a semiconductor substrate and, particularly, on a semiconductor photo mask.
A semiconductor photo mask is used for exposing a circuit pattern on a wafer in a manufacturing process of a semiconductor device, a liquid crystal display device, etc. A semiconductor photo mask is a transparent substrate on that a minute pattern film, which is an opaque film, is formed. When the pattern film is formed on the substrate, two types of defects may occur; a white defect or clear defect, at which a part of the pattern film is lost, and a black defect or an opaque defect, at which the substrate is unnecessarily covered with film.
For correcting white defects on a semiconductor photo mask, laser CVD (chemical vapor deposition) is available. According to laser CVD, laser light is irradiated at the white defects on the substrate in material gas including chromium. Laser light thermally resolves material gas around the white defects and as a result, film including chromium grows over the white defects.
An example of conventional technique for correcting white defects on a semiconductor photo mask is Japanese Patent number (JP-B) 3036687 which is hereinafter referred to as first conventional technique. According to the first conventional technique, an optical system for laser irradiator/laser microscope, which includes laser light source and a unit for irradiation/observation, is arranged above the substrate set in material gas. The optical system downward irradiates laser light at white defects on the substrate. The optical system shapes laser light by its slit and projects a pattern on the substrate. As a result, the optical system corrects white defects precisely.
On the other hand, black defects, namely unnecessary part of pattern film, is conventionally vaporized to correct a semiconductor photo mask by irradiating laser light on the part.
Japanese Unexamined Patent Publication (JP-A) number H7-104459, namely 104459/1995 is titled “Method and apparatus for correcting defects on a photo mask”, which is hereinafter referred to as second conventional technique. According to the second conventional technique, first, pattern film is printed on one surface of transparent substrate. Next, the substrate is laid on the surface. Then, laser light is irradiated from the other surface through the substrate at black defects to vaporize the black defects. Since the former surface faces downward, particles generated when the black defects are vaporized drop down, and therefore, are not stuck on the former surface.
According to the first conventional technique, laser light is irradiated down at white defects on a substrate to form the pattern film covering the white defects. Laser CVD generates particles from decomposed material gas. The particles in the air fall down and are struck again around the area at which laser light has been irradiated on the substrate. From the struck particles, CVD forms unnecessary film on the substrate. As a result, edge of the pattern film swells up on border of the mask pattern. Compared with the second conventional technique, the first conventional technique has less precision.
According to the second conventional technique, on the other hand, the surface of the substrate on which laser light is irradiated faces downward, and consequently, most particles generated by laser light fall down. However, 0.1 μm or smaller particles often stick on the substrate again and cause a decline of transmittance around the area at which laser light has been irradiated. The decline of transmittance seriously affects the newest apparatus for correcting photo mask in 0.13 μm rule, which corresponds to 0.5 μm rule on its original photo mask.